Gergo P. Szakmany

Response Increase of IR Antenna-Coupled Thermocouple Using Impedance Matching

The response of a bowtie antenna-coupled thermocouple operating at 10.6 μm is studied for varying lengths of a transmission line, which connects the antenna to the thermocouple and functions as an impedance-matching element. Peaks in the response are observed for several lengths of transmission line. Most notably, the response of a device with a transmission line length of 1.3 μm is increased 2.4 fold when compared to the device without the transmission line. The analytical response of a microwave circuit describing the detector is in agreement with the measurements, indicating that the increases in the response are caused by an improved impedance match between the antenna and thermocouple facilitated by the transmission line. This experiment demonstrates for the first time impedance matching principles applied to infrared antenna-coupled thermal detectors.

Antenna-Coupled Nanowire Thermocouples for Infrared Detection

Unbiased, uncooled, and frequency-selective antenna-coupled nanowire thermocouples have been fabricated out of different metal combinations and characterized for infrared detection. The relative Seebeck coefficient of the nanowire thermocouples was measured with a characterization platform, which is colocated on the same chip as the detectors. The area of the hot junction of the nanowire thermocouple is approximately 75 nm × 75 nm. The antenna-coupled thermocouples show polarization dependence with a maximum normalized detectivity (D*) of 1.94 × 105 cm · √Hz/W.

Single-Metal Nanoscale Thermocouples

We study the generation of thermoelectricity by nanoscale thermocouples (TCs) formed from a single layer of metal with cross-sectional discontinuity. Typically, a TC is formed when a second conductor is inserted between two sections of a first conductor forming two junctions situated at different temperatures. Here, we investigate the behavior of TCs formed not of two conductors but rather nanowires of the same metal of two cross-sectional areas. Monometallic TCs were constructed from a lithographically defined nanowire having one abrupt variation in width along its length, and tested at room temperature; these structures exploit a change in Seebeck coefficient that is present at these size scales. To investigate the thermoelectric properties of such “shape-engineered” TCs, nanoscale heaters were employed to set the local temperatures. Temperature profiles at the hot and cold junctions of the TCs were determined both by simulations and experiments. Results demonstrate that the magnitude of the open-circuit voltage, and hence the relative Seebeck coefficient, is a function of the parameters of the variations in the segment widths. The fabrication complexity of such shape-engineered monometallic nanowire TCs is greatly reduced compared to that of conventional bimetallic TCs, and could be mass-produced using simpler manufacturing techniques.

Nanowire Thermocouple Characterization Platform

Thermocouples fabricated out of nanowires possess a high spatial and temporal resolution. Due to their small size, nanowires exhibit different physical properties from their bulk counterparts. One of these properties, the Seebeck coefficient, specifies how well the thermocouple converts a temperature gradient into an open-circuit voltage. We have developed a characterization platform, with which the Seebeck coefficient of nanowires can be measured as required for the calibration of nanowire thermocouples and optimization of their fabrication process.

Rectennas Revisited

In the late 1960s, a new concept was proposed for an infrared absorbing device called a “rectenna” that, combining an antenna and a nanoscale metal-insulator-metal diode rectifier, collects electromagnetic radiation in the terahertz regime, with applications as detectors and energy harvesters. Previous theories hold that the diode rectifies the induced terahertz currents. Our results, however, demonstrate that the Seebeck thermal effect is the actual dominant rectifying mechanism. This new realization that the underlying mechanism is thermal-based, rather than tunneling-based, can open the way to important new developments in the field, since the fabrication process of rectennas based on the Seebeck effect is far simpler than existing processes that require delicate tunnel junctions. We demonstrate for the first time the fabrication of a rectenna array using an efficient parallel transfer printing process featuring nearly one million elements.

Nanoantenna Infrared Detectors

This project focuses on devices that can be used for detection of thermal or long-wave infrared radiation, which is a frequency range for which developing detectors is of special interest. Objects near 300 K, such as humans and animals, emit radiation most strongly in this range, and absorption is relatively low in the LWIR atmospheric window between 8 and 14 μm. These facts provide motivation to develop detectors for use in this frequency range that could be used for target detection, tracking, and navigation in autonomous vehicles. The devices discussed in this chapter, referred to as dipole antenna-coupled metal-oxide-metal diodes (ACMOMDs), feature a half-wavelength antenna that couples electromagnetic radiation to a metal-oxide-metal (MOM) diode, which acts as a nonlinear junction to rectify the signal. These detectors are patterned using electron beam lithography and fabricated with shadow evaporation metal deposition. Along with offering CMOS compatible fabrication, these detectors provide high-speed and frequency-selective detection without biasing, a small pixel footprint, and full functionality at room temperature without cooling. The detection characteristics can be tailored to provide for multi-spectral imaging in specific applications by modifying device geometries. This chapter gives a brief introduction to currently available infrared detectors, thereby providing a motivation for why ACMOMDs were chosen for this project. An overview of the metal-oxide metal diode is provided, detailing principles of operation and detection. The fabrication of ACMOMDs is described in detail, from bonding pad through device processes. Direct-current current–voltage characteristics of symmetrical and asymmetrical antenna diodes are presented. An experimental infrared test bench used for determining the detection characteristics of these detectors is detailed, along with the figures of merit which have been measured and calculated. The measured performance of fabricated ACMOMDs is presented, including responsivity, noise performance, signal-to-noise ratio, noise-equivalent power, and normalized detectivity. The response as a function of infrared input power, polarization dependence, and antenna-length dependence of these devices is also presented.

Novel Nanoscale Single-Metal Polarization-Sensitive Infrared Detectors

We report a vastly simplified approach to infrared (IR) detection based on a single-metal nanostructure. This integrated structure contains a dipole antenna and a thermocouple (TC). The antenna provides wavelength selectivity, and the TC provides the conversion from optical to electrical signals. Moreover, the same nanowire structure serves both as the receiving element (antenna) and as the rectifying element (TC), yielding a highly integrated detector system. This study exploits a newly discovered thermoelectric effect in single-metal nanostructures with cross-sectional discontinuities to affect the TC functionality. In order to optimize the IR response, devices with various antenna lengths are simulated and fabricated. Both simulations and experiments demonstrate that the locations of the hot and cold TC junctions reverse as the polarization of the incident IR radiation changes, which results in reversal of the output signal polarity. The fabrication complexity of these single-metal devices is greatly reduced compared to that of other IR detector approaches, and their unique polarization-dependent response makes them attractive for new classes of IR systems.

A Nanostructured Long-Wave Infrared Range Thermocouple Detector

Infrared (IR) detectors for the long-wave infrared range (LWIR) based on nanostructured thermocouples exhibit response times in the order of picoseconds and will open new areas of applications in THz broadband communication systems. We show experimental evidence for a nonzero relative Seebeck coefficient between the narrow and the wide wire segments of single-material thermocouples, and we investigate the thermal dynamics of nano-thermocouples for a substrate heated by a nanostructured thermocouple patch. Due to its low thermal capacity a mesoscopic thermocouple is an extremely fast square-law detector with cutoff frequencies up to several hundred GHz.

High-Speed Antenna-Coupled Terahertz Thermocouple Detectors and Mixers

An antenna-coupled nanothermocouple (ACNTC) is an integrated structure consisting of a dipole nanoantenna and a nanothermocouple (NTC). ACNTCs are excellently suited as polarization-sensitive detectors and mixers for the long-wavelength far-infrared range around 30 THz. Radiation collected by the integrated nanoantenna and fed into the hot junction creates a temperature difference between the hot and cold junctions of the thermocouple, which results in open-circuit voltage due to the Seebeck effect. Due to the geometry-dependence of the Seebeck coefficient in nanowires, we realize single-metal ACNTCs. The fundamentals of single-metal NTCs are discussed. The thermal dynamics of NTCs is investigated showing that NTCs could exhibit mixer and detector intermediate frequency and low frequency cutoffs beyond 100 GHz. We provide experimental evidence of ACNTCs.

Comment on “Unexpected size effect in the thermopower of thin-film stripes” [J. Appl. Phys. 110, 083709 (2011)]

In a recent article, Sun et al. [J. Appl. Phys. 110, 083709 (2011)] claim to measure a size-dependent thermoelectric effect in a micron-scale single-metal thermocouple. In this Comment, we demonstrate that the observed phenomenon is not due to a size-dependent Seebeck effect as claimed, but is rather wire-size-dependent heat transport that causes unequal heating at the bonding pads. As a result, the bonding pads are at two different temperatures, and the observed voltage corresponds to a thermoelectric effect of a parasitic thermocouple formed between their metal structure and the bonding-pad wires. We provide simulations and suggest a control experiment based on their structure that supports our contention that the observation depends on width-dependent heat transport in the wires.